Dynamic calibration of axial accelerometers and magnetometers

ABSTRACT

A method to perform a drilling operation. The method includes calibrating an axial survey sensor of a bottom hole assembly (BHA) by obtaining, from the axial survey sensor, a data log as a first function of azimuthal angle within a borehole, generating, by a computer processor of the BHA and using a pre-determined algorithm, a fitted curve as a second function of the azimuthal angle, wherein the fitted curve is generated based on the data log to represent a calibration error of the axial survey sensor, and extracting, by the computer processor of the BHA, a calibration parameter from the fitted curve. Accordingly, the drilling operation is performed using at least the axial survey sensor based on the calibration parameter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 61/858,806, filed on Jul. 26, 2013,and entitled “Dynamic Calibration of Axial Accelerometers andMagnetometers,” which is hereby incorporated by reference.

BACKGROUND

Tri-axial accelerometers and magnetometers are widely used in oil andgas well characterization to measure the components of gravitational gand the earth magnetic field intensity B in a tool coordinate system. Byconvention, the tool z-axis is the long axis of the tool, whichcorresponds to the borehole axis. The magnetometers and accelerometersare calibrated in a laboratory for scale factor, bias, and misalignment.These sensors are calibrated for temperatures up to their specifications(e.g., up to 150° C., 175° C., 200° C., etc). In addition, the validityof calibration is also periodically checked at room temperature.

While such laboratory calibration methods of sensors' scale factor,bias, misalignment, temperature work reasonably well downhole, thesesensors still suffer from systematic errors due to the aging effect ofsensors (e.g., sensor characteristic change over months fromhigh-temperature and high-shock exposures), internal physical componentchange/damage, etc. Generally, such systematic errors could be observedin the field in-between calibration periods (every 2-3 months to 6months). For example, bias errors greatly affect the results ofaccelerometers and magnetometers at near-vertical inclination and/or atdirections near the magnetic dip axis. Axial sensor misalignment affectsthe measurement consistency and precision (resulting in misalignmenterrors) at near-horizontal inclination and/or at directions close tomagnetic east/west, where axial accelerometer and/or magnetometerreading are very small. The aforementioned systemic errors, bias errors,and misalignment errors are referred to as calibration error throughoutthis disclosure.

SUMMARY

In general, in one aspect, the invention relate to a method to perform adrilling operation. The method includes (i) calibrating an axial surveysensor of a bottom hole assembly (BHA) by obtaining, from the axialsurvey sensor, a data log as a first function of azimuthal angle withina borehole, wherein the axial sensor comprises at least one selectedfrom a group consisting of an accelerometer and a magnetometer, whereinthe axial sensor is a part of at least one selected from a groupconsisting of a MWD tool, a LWD tool, a downhole imaging tool, adownhole motor, and a rotary steerable tool, and wherein the azimuthalangle is based on at least one selected from a group consisting of agravity toolface, a magnetic toolface, and a gyro toolface, generating,by a computer processor of the BHA and using a pre-determined algorithm,a fitted curve as a second function of the azimuthal angle, wherein thefitted curve is generated based on the data log to represent acalibration error of the axial survey sensor, and extracting, by thecomputer processor of the BHA, a calibration parameter from the fittedcurve, and (ii) performing the drilling operation using at least theaxial survey sensor based on the calibration parameter, wherein thecalibrating is performed during at least one selected from a groupconsisting of when the axial survey sensor is rotating and when theaxial survey sensor is not rotating.

Other aspects of the invention will be apparent from the followingdetailed description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

The appended drawings illustrate several embodiments of dynamiccalibration of axial accelerometers and magnetometers and are not to beconsidered limiting of its scope, for dynamic calibration of axialaccelerometers and magnetometers may admit to other equally effectiveembodiments.

FIG. 1.1 is a schematic view of a wellsite depicting a drillingoperation in which one or more embodiments of dynamic calibration ofaxial accelerometers and magnetometers may be implemented.

FIG. 1.2 shows a system for dynamic calibration of axial accelerometersand magnetometers in accordance with one or more embodiments.

FIGS. 2.1-2.2 shows a flowchart of a method for dynamic calibration ofaxial accelerometers and magnetometers in accordance with one or moreembodiments.

FIGS. 3.1-3.2 depict examples of dynamic calibration of axialaccelerometers and magnetometers in accordance with one or moreembodiments.

FIG. 4 depicts a computer system using which one or more embodiments ofdynamic calibration of axial accelerometers and magnetometers may beimplemented.

DETAILED DESCRIPTION

Aspects of the present disclosure are shown in the above-identifieddrawings and described below. In the description, like or identicalreference numerals are used to identify common or similar elements. Thedrawings are not necessarily to scale and certain features may be shownexaggerated in scale or in schematic in the interest of clarity andconciseness.

Dynamic survey refers to obtaining measurements from a survey sensorwhile the survey sensor is continuously rotating with the drill string.Static survey refers to obtaining measurements from the survey sensorwhile the survey sensor stops continuous rotation with the drill string.In the static survey, there may still be rotation of the sensor platform(and therefore the sensors mounted thereon) in some situations. Forexample, during normal condition in the static survey, theroll-stabilized platform may be stationary in a non-rotating phase orrotate at less than 5 rpm in a rotating phase, a slowly-rotating housingor a slowly-rotating sleeve housing may rotate at less than 20revolutions per hour (rph). In one or more embodiments of dynamiccalibration of axial accelerometers and magnetometers, the survey sensoris an axial magnetometer and/or accelerometer (generally referred to asan axial survey sensor), and the axial magnetometer and/or accelerometermeasurements include information related to gravitational accelerationand the earth magnetic field.

Aspects of the present disclosure include a method, system, and computerreadable medium for improving axial survey sensor measurements, such asaxial magnetometer and accelerometer measurements. The methoddynamically corrects for axial misalignment errors downhole. In otherwords, the axial misalignment errors are computed during a dynamicsurvey. The computed axial misalignment errors are applied back tosubsequent static and dynamic surveys to improve the consistency andprecision of inclination, azimuth, total magnetic field, total gravityfield, and magnetic dip angle computations in both static and dynamicsurveys. Further, the method is used in a rotary-steerable system toincrease automated steering accuracy by improving static and dynamicsurvey consistency and precision.

FIG. 1.1 is a schematic view of a wellsite (100) depicting a drillingoperation. The wellsite (100) includes a drilling system (311) and asurface unit (334). In the illustrated embodiment, a borehole (313) isformed by rotary drilling in a manner that is well known. Those ofordinary skill in the art given the benefit of this disclosure willappreciate, however, that the present invention also finds applicationin drilling applications other than conventional rotary drilling (e.g.,mud-motor based directional drilling), and is not limited to land-basedrigs.

The drilling system (311) includes a drill string (315) suspended withinthe borehole (313) with a drill bit (310) at its lower end. The drillingsystem (311) also includes the land-based platform and derrick assembly(312) positioned over the borehole (313) penetrating a subterraneanformation (F). The assembly (312) includes a rotary table (314), kelly(316), hook (318) and rotary swivel (319). The drill string (315) isrotated by the rotary table (314), energized by means not shown, whichengages the kelly (316) at the upper end of the drill string. The drillstring (315) is suspended from hook (318), attached to a traveling block(also not shown), through the kelly (316) and a rotary swivel (319)which permits rotation of the drill string relative to the hook.

The drilling system (311) further includes drilling fluid or mud (320)stored in a pit (322) formed at the well site. A pump (324) delivers thedrilling fluid (320) to the interior of the drill string (315) via aport in the swivel (319), inducing the drilling fluid to flow downwardlythrough the drill string (315) as indicated by the directional arrow.The drilling fluid (320) exits the drill string (315) via ports in thedrill bit (310), and then circulates upwardly through the region betweenthe outside of the drill string (315) and the wall of the borehole(313), called the annulus (326). In this manner, the drilling fluid(320) lubricates the drill bit (310) and carries formation cuttings upto the surface as it is returned to the pit (322) for recirculation.

The drill string (315) further includes the BHA (330), near the drillbit (310) (in other words, within several drill collar lengths from thedrill bit). The BHA (330) includes capabilities for measuring,processing, and storing information, as well as communicating with thesurface unit. The BHA (330) further includes drill collars (328) forperforming various other measurement functions. In particular, the BHA(330) includes the dynamic calibration unit (200).

Sensors (S) are located about the wellsite to collect data, may be inreal time, concerning the operation of the wellsite, as well asconditions at the wellsite. The sensors (S) may also have features orcapabilities, of monitors, such as cameras (not shown), to providepictures of the operation. Surface sensors or gauges (S) may be deployedabout the surface systems to provide information about the surface unit,such as standpipe pressure, hook load, depth, surface torque, rotaryrpm, among others. Downhole sensors or gauges (S) are disposed about thedrilling tool and/or wellbore to provide information about downholeconditions, such as wellbore pressure, weight on bit, torque on bit,direction, inclination, collar rpm, tool temperature, annulartemperature and toolface, among others. Multiple downhole sensors (S)may be located at different positions on BHA (330), such as sensor (201)and sensor (202). The information collected by the sensors and camerasis conveyed to the various parts of the drilling system and/or thesurface unit (334).

The drilling system (311) is operatively connected to the surface unit(334) for communication therewith. The BHA (330) is provided with acommunication subassembly (352) that communicates with the surface unit.The communication subassembly (352) is adapted to send signals to andreceive signals from the surface using mud pulse telemetry. Thecommunication subassembly (352) may include, for example, a transmitterthat generates a signal, such as an acoustic or electromagnetic signal,which is representative of the measured drilling parameters. It will beappreciated by one of ordinary skill in the art that a variety oftelemetry systems may be employed, such as mud pulse telemetry, wireddrill pipe, electromagnetic or other known telemetry systems.

Typically, the wellbore is drilled according to a drilling plan that isestablished prior to drilling. The drilling plan typically sets forthequipment, pressures, trajectories and/or other parameters that definethe drilling process for the wellsite. The drilling operation may thenbe performed according to the drilling plan. However, as information isgathered, the drilling operation may deviate from the drilling plan.Additionally, as drilling or other operations are performed, thesubsurface conditions may change. The earth model may also be adjustedas new information is collected.

Although the subterranean assets are not limited to hydrocarbons such asoil, throughout this document, the terms “oilfield” and “oilfieldoperation” may be used interchangeably with the terms “field” and “fieldoperation” to refer to a site where any type of valuable fluids can befound and the activities required for extracting them. The terms mayalso refer to sites where substances are deposited or stored byinjecting them into the surface using boreholes and the operationsassociated with this process. Further, the term “field operation” refersto a field operation associated with a field, including activitiesrelated to field planning, wellbore drilling, wellbore completion,and/or production using the wellbore.

FIG. 1.2 is a schematic view of the BHA (330) with more details. In oneor more embodiments of the invention, one or more of the modules andelements shown in FIG. 1.2 may be omitted, repeated, and/or substituted.Accordingly, embodiments of dynamic calibration of axial accelerometersand magnetometers should not be considered limited to the specificarrangements of modules shown in FIG. 1.2.

As shown in FIG. 1.2, the BHA (330) includes a sensor (202) and adynamic calibration unit (200) for calibrating the sensor (202). In oneor more embodiments, the sensor (202) is an axial accelerometer or anaxial magnetometer, which may be part of a measurements-while-drilling(MWD) tool, a logging-while-drilling (LWD) tool, a downhole imagingtool, a downhole motor, or a rotary steerable tool. In one or moreembodiments, the axial accelerometer or axial magnetometer may belocated on a rotary steerable platform (e.g., rotating with the drillbit up to 250 rotations-per-minute (rpm) during drilling),roll-stabilized platform (e.g., rotating at less than 5 rpm), anon-rotating platform, a slowly-rotating housing (e.g., rotating at lessthan 20 rotations-per-hour (rph)), or a slowly-rotating sleeve housingwith a controlled rotation speed (e.g., rotating at less than 20rotations-per-hour (rph)). Examples of these platforms/housings aredisclosed in U.S. Pat. Nos. 5,265,682, 5,353,884, 6,427,783, and/or7,950,473. Further, the dynamic calibration unit (200) includes anazimuthal dependency analyzer (221), a calibration module (222), and arepository (210). In one or more embodiments, one or both of theazimuthal dependency analyzer (221) and the calibration module (222) isa software module executing on a computer processor (not shown) of theBHA (330), a hardware module (e.g., a digital circuitry, an analogcircuitry, a programmable logic device, a field-programmable gate array,or a combination) installed on the BHA (330), or a combination thereof.The repository (210) may be a semiconductor based data storage device, arotating disk based data storage device, or other suitable computer datastorage. In particular, at least a portion of the repository (330) issubject to limitations (e.g., limited capacity) induced by downholeconditions, such as extreme temperature and/or shock/vibration. In oneor more embodiments, a portion of the repository (330) may be located inthe surface unit (334). As shown in FIG. 1.2, the repository (210) maystore data log generated by the axial survey sensor (202), such as anazimuthal dependent data log (211). Further, the repository (210) mayalso store intermediate or end results of the dynamic calibration unit(200), such as a fitted curve (212), a calibration parameter (213), anda corrected data log (214).

In one or more embodiments, the azimuthal dependency analyzer (221) isconfigured to obtain, from the axial survey sensor (202), the azimuthaldependent data log (211) as a function of azimuthal angle within theborehole (313). In one or more embodiments, the azimuthal angle is basedon a gravity toolface, a magnetic toolface, or a gyro toolface. In oneor more embodiments, the azimuthal dependent data log (211) includesinformation related to gravitational acceleration and the earth magneticfield. While the gravitational acceleration and the earth magnetic fieldare substantially independent of the orientation of the axial surveysensor (202), the downhole data generated by the axial survey sensor(202) includes azimuthal angle dependent components, such as anazimuthal angle dependent calibration error.

Using a pre-determined algorithm, the azimuthal dependency analyzer(221) is further configured to generate a fitted curve (212) based onthe azimuthal dependent data log (211) to represent a calibration errorof the axial survey sensor (202). Specifically, the fitted curve (212)is also a function of the azimuthal angle. Accordingly, the calibrationparameter (213) is extracted from the fitted curve by the azimuthaldependency analyzer (221). In one or more embodiments, the calibrationis performed when the axial survey sensor (202) is rotating or when theaxial survey sensor (202) is not rotating.

In one or more embodiments, the calibration module (222) is configuredto calibrate the sensor (202) using the calibration parameter (213). Forexample, the corrected (i.e., calibrated) data log (214) may begenerated by combining the azimuthal dependent data log (211) with acorrection to the calibration error. As noted above, the calibrationerror is represented by the fitted curve (212). In one or moreembodiments, the correction to the calibration error is determined atleast based on the calibration parameter (213) that is extracted fromthe fitted curve (212). In one or more embodiments, multiple calibrationparameters are extracted from the fitted curve (212) and used todetermine the correction to the calibration error. In one or moreembodiments, the calibration module (222) is configured to calibrate thesensor (202) in real time. In such embodiments, the fitted curve (212),the calibration parameter (213), and the corrected data log (214) areall generated from the same azimuthal dependent data log (211). Inparticular, they are generated within a pre-determined time period(e.g., 1 minute) after the azimuthal dependent data log (211) isobtained from the sensor (202). In one or more embodiments, thecalibration module (222) is configured to calibrate the sensor (202)simultaneously with the drilling operation. For example, while a LWDtool with the sensor (202) installed thereon are rotating, themeasurement of the navigational parameters (inclination, azimuth,gravity toolface, etc) occurs. At the same time, the dynamic calibrationis performed by the calibration module (222). For example, the dynamiccalibration of axial sensor occurs every minute, and at the end of thisperiod, the correction factor is identified and a misalignmentcorrection is applied for the next one minute. This process is repeated;and therefore, a new correction factor is determined every minute anddynamically compensates the calibration error downhole.

In summary, the azimuthal dependency analyzer (221) dynamicallycalculates axial misalignment errors of the sensor (202) downhole (whiledrilling). The computed axial misalignment errors are applied back tostatic surveys by the calibration module (222). As a result, theconsistency and precision of inclination, azimuth, total magnetic field,total gravity field, magnetic dip angle computations are improved bothin static and dynamic surveys. As is known in the art, static survey istaken when the tool is not rotating (i.e., not drilling) and dynamic (orcontinuous) survey is taken while the tool is drilling ahead.

In one or more embodiments, the drilling operation is based on one ormore of MWD, LWD, LWD imaging, and rotary steerable tools with the axialsurvey sensor (202) packaged in rotating, slowly-rotating, orsemi-rotating (roll-stabilized) housings. Accordingly, the drillingoperation is improved because of more accurate data log from the axialsurvey sensor (202) based on the calibration parameter (213). Forexample, the drilling operation may use a rotary-steerable system whereautomated steering accuracy is enhanced by improving static and dynamicsurvey consistency and precision.

Examples of generating the fitted curve (212), extracting thecalibration parameter (213), and other aspects of dynamic calibrationfor axial accelerometers and magnetometers are described in reference toFIGS. 3.1 and 3.2 below.

FIG. 2.1 shows a method flowchart in accordance with one or moreembodiments of the invention. In one or more embodiments of theinvention, the method of FIG. 2.1 may be practiced using the BHA (330)described in reference to FIG. 1.2 above. In one or more embodiments ofthe invention, one or more of the Blocks shown in FIG. 2.1 may beomitted, repeated, and/or performed in a different order than that shownin FIG. 2.1. Accordingly, the specific arrangement of Blocks shown inFIG. 2.1 should not be construed as limiting the scope of the invention.

The method flowchart depicted in FIG. 2.1 relates to performing adrilling operation. Specifically, Blocks 201, 202, and 203 relate tocalibrating an axial survey sensor of a bottom hole assembly (BHA),while Blocks 204 and 205 relate to performing the drilling operationbased on corrected downhole data generated using the calibrated axialsurvey sensor.

Initially in Block 201, a data log is obtained from the axial surveysensor as a first function of azimuthal angle within a borehole. In oneor more embodiments, the data log corresponds to a range of azimuthalangles of the axial survey sensor within the borehole. In particular,the data log includes a collection of downhole data generated by theaxial survey sensor oriented at multiple azimuthal angles throughout theazimuthal range within the borehole.

In one or more embodiments, the axial survey sensor includes anaccelerometer and/or a magnetometer. Specifically, the axial sensor is apart of a MWD tool, a LWD tool, a downhole imaging tool, a downholemotor, and/or a rotary steerable tool. In addition, the azimuthal angleis based on a gravity toolface, a magnetic toolface, or a gyro toolface.In one or more embodiments, the data log includes information related togravitational acceleration and the earth magnetic field. While thegravitational acceleration and the earth magnetic field aresubstantially independent of the orientation of the axial survey sensor,the downhole data generated by the axial survey sensor includesazimuthal angle dependent components, such as the calibration error thatis azimuthal angle dependent.

In one or more embodiments, a drill string is continuously rotatingwithin the borehole during the calibration, referred to as a dynamiccalibration. In other words, the dynamic calibration is performed usinga dynamic survey when the axial survey sensor is continuously rotatingwith the drill string. In such embodiments, each data item of the datalog is generated by the axial survey sensor as the azimuthal angle ofthe axial survey sensor increments during one or more revolutions of thedrill string.

In one or more embodiments, the drill string stops continuous rotationwithin the borehole during the calibration, referred to as a staticcalibration. For example, the static calibration may be performed usingmultiple static surveys by sequentially orienting the axial surveysensor at multiple stationery azimuthal angles. In such embodiments,each data item of the data log is generated by the axial survey sensor,oriented at a particular stationery azimuthal angle, before the axialsurvey sensor is incrementally turned to another stationery azimuthalangle for generating the subsequent data item in the data log.

In one or more embodiments, the data log may be obtained using anaccelerometer or magnetometer located on various sensor mountingplatforms, such as a rotary steerable platform, a roll-stabilizedplatform, a non-rotating platform, or a slowly-rotating housing of theBHA.

In Block 202, a fitted curve is generated by a computer processor of theBHA and using a pre-determined algorithm. In particular, the fittedcurve is generated as a second function of the azimuthal angle.Specifically, the fitted curve is generated based on the data log torepresent a calibration error of the axial survey sensor. In one or moreembodiments, the fitted curve is a mathematical SINE function of theazimuthal angle, where the period of the SINE function corresponds toone complete revolution of the axial survey sensor within the borehole.

In Block 203, a calibration parameter is extracted from the fitted curveby the computer processor of the BHA. In one or more embodiments, thecalibration parameter includes a peak-to-peak amplitude and a phaseshift angle of the SINE function.

In Block 204, during either a dynamic survey or a static surveysubsequent to the calibration, azimuthal dependent downhole data iscorrected in real time based on the calibration parameter to generatecorrected downhole data. In particular, the azimuthal dependent downholedata includes a measurement generated by the axial survey sensororiented at a particular azimuthal angle. The azimuthal angle associatedwith the azimuthal dependent downhole data is then used to predict thecalibration error based on the calibration parameter. For example, theazimuthal angle and the calibration parameter are used as inputs to theSINE function to generate an output that represents the predictedcalibration error at the particular azimuthal angle. Accordingly, thepredicted calibration error is removed (e.g., subtracted) from theazimuthal dependent downhole data to generate the corrected downholedata.

In Block 205, the drilling operation is performed using at least theaxial survey sensor based on the calibration parameter. Specifically,the drilling operation is performed based on the corrected downhole datathat is calibrated in real time.

Examples of generating the fitted curve, extracting the calibrationparameter, and other aspects of calibrating the axial survey sensor aredescribed in reference to FIGS. 3.1 and 3.2 below.

FIG. 2.2 shows a method flowchart in accordance with one or moreembodiments of the invention. In one or more embodiments of theinvention, the method of FIG. 2.2 may be practiced using the BHA (330)described in reference to FIG. 1.2 above. In one or more embodiments ofthe invention, one or more of the Blocks shown in FIG. 2.2 may beomitted, repeated, and/or performed in a different order than that shownin FIG. 2.2. Accordingly, the specific arrangement of Blocks shown inFIG. 2.2 should not be construed as limiting the scope of the invention.

The method flowchart depicted in FIG. 2.2 relates to performing adrilling operation as directed by an output generated by an axial surveysensor of a bottom hole assembly (BHA). In one or more embodiments, theoutput of the axial survey sensor is automatically corrected during adynamic survey to eliminate calibration error without performing aseparate calibration of the axial survey sensor. In one or moreembodiments, the output of the axial survey sensor is automaticallycorrected using multiple static surveys to eliminate calibration errorwithout performing a separate calibration of the axial survey sensor.

In Block 211, a data log is obtained from the survey sensor, as afunction of azimuthal angle within a borehole. In one or moreembodiments, the data log corresponds to one or more completerevolutions of the axial survey sensor within the borehole. Inparticular, the data log includes a collection of downhole datagenerated by the axial survey sensor oriented at multiple azimuthalangles throughout each revolution within the borehole.

In one or more embodiments, a drill string is continuously rotatingwithin the borehole when the data log is obtained during a dynamicsurvey. In other words, the data log is obtained during the dynamicsurvey when the axial survey sensor is continuously rotating with thedrill string. In such embodiments, each data item of the data log isgenerated by the axial survey sensor as the azimuthal angle of the axialsurvey sensor increments during one or more revolutions of the drillstring.

In one or more embodiments, the drill string stops continuous rotationwithin the borehole when the data log is obtained in multiple staticsurveys. For example, the data log may be obtained using multiple staticsurveys by sequentially orienting the axial survey sensor at multiplestationery azimuthal angles. In such embodiments, each data item of thedata log is generated by the axial survey sensor, oriented at aparticular stationery azimuthal angle, before the axial survey sensor isincrementally turned to another stationery azimuthal angle forgenerating the subsequent data item in the data log.

In one or more embodiments, the data log may be obtained using anaccelerometer or magnetometer located on various sensor mountingplatforms, such as a rotary steerable platform, a roll-stabilizedplatform, a non-rotating platform, or a slowly-rotating housing of theBHA.

In Block 212, during either the dynamic survey or the multiple staticsurveys described in Block 211 above, a corrected value of the data logis generated by a computer processor, using a pre-determined algorithm.The computer processor may be part of the BHA or at a processingfacility at the surface (e.g. a surface unit). Specifically, acalibration error of the axial survey sensor is eliminated from thecorrected value (referred to as the corrected downhole data) based onthe pre-determined algorithm. In one or more embodiments, thecalibration error is represented or predicted by the pre-determinedalgorithm as a mathematical SINE function of the azimuthal angle, wherethe period of the SINE function corresponds to one complete revolutionof the axial survey sensor within the borehole. Because the data logcorresponds to complete revolution(s), i.e., complete period(s) of theSINE function, the calibration errors associated with each and everydata item of the data log are mathematically cancelled out by summingall data items of the data log. Accordingly, the corrected downhole datais generated as the summation of all data items of the data log. In oneor more embodiments, the corrected downhole data is further normalizedbased on a normalization factor specified in the pre-determinedalgorithm.

In Block 213, the drilling operation is performed based on the correcteddownhole data that is automatically calibrated as described above.Examples of generating the corrected downhole data that is automaticallycalibrated are described in reference to FIGS. 3.1 and 3.2 below.

FIGS. 3.1-3.2 depict examples of dynamic calibration of axialaccelerometers and magnetometers in accordance with one or moreembodiments. In one or more embodiments of the invention, the exampleshown in FIGS. 3.1-3.2 may be practiced using the BHA (330) described inreference to FIG. 1.2 and the method flowcharts described in referenceto FIGS. 2.1 and 2.2.

By convention, the gravitational field is considered to be positivepointing downward (i.e., toward the center of the earth), while themagnetic field is considered to be positive pointing towards magneticnorth. Moreover, also by convention, the y-axis is aligned with thetoolface reference axis. Specifically, the gravity toolface (GTF) equalszero when the y-axis is along the opposite direction of the gravityfield vector and the magnetic toolface (MTF) equals zero when the y-axisis pointing towards magnetic north. Accordingly, the magnetic toolfaceMTF is projected in the xy plane and may be represented mathematicallyas: tan(MTF)=B_(x)/B_(y). Likewise, the gravity toolface GTF may berepresented mathematically as: tan(GTF)=A_(x)/A_(y). Here, (B_(x),B_(y)) and (A_(x), A_(y)) represent measurement results of the axialaccelerometer and magnetometer, respectively.

An example of axial misalignment can be observed in downhole recordeddata shown in FIG. 3.1. Specifically, FIG. 3.1 shows axial magnetometerdata with magnetic toolface dependency (or azimuthal dependency). On topand bottom plots in FIG. 3.1, X-axis is discretized magnetic toolface(every 1 degree), having 360 azimuthal sectors. Y-axis on the top plotshows the number of samples observed on each discrete toolface over a2-minute period (i.e., sampled at 100 Hz and there are 12000 samples).The bottom plot shows the mean axial magnetic field strength for eachdiscrete toolface (in this case, every 1 degree). As can be seen, theaxial magnetic strength is highly correlated to the magnetic toolfaceangle, even though noise appears due to statistical errors (on average33.3 samples per discrete toolface).

Axial misalignment correction may be made by characterizing its toolfacedependency on axial measurement (either B_(z) or A_(z)). The correctionalgorithm follows the following steps:

1) Compute mean axial magnetic/gravity field strength for each discretetoolface

2) Fit a sine wave to the discrete data points.

3) Store the scale factor, offset and phase information of the fittedsine wave in memory

In the while-drilling axial misalignment method, the corrected axialvalue is the “offset” of the fitted sine wave. Alternatively, thearithmetic mean of all the averaged discrete data points (computed inStep 1) may be used.

4) The memory-stored sine-wave parameters (scale factor, offset, andphase) can be applied back to the next static survey in order toincrease survey consistency and precision.

FIG. 3.2 shows an example of axial magnetometer misalignment correction.In FIG. 3.2, x-axis is discretized magnetic toolface (every 10 degree),having 36 azimuthal sectors. The top plot shows the number of samplesobserved at each discrete magnetic toolface (every 10 degrees). Thesesamples may represent axial magnetometer measurements obtained during adynamic survey or obtained during multiple static surveys.

The middle plot shows the mean axial magnetic field strength for eachdiscrete toolface, superimposed with a fitted sine wave (shown withcrosses (+++)). In one scenario, based on the method flowchart depictedin FIG. 2.1 above, the sine wave is used to extract calibrationparameter(s) that are in turn used to correct subsequent axialmagnetometer measurements. In another scenario, based on the methodflowchart depicted in FIG. 2.2 above, the mean axial magnetic fieldstrength for each discrete toolface is included in a summation to cancelout the sine wave, thus generating the corrected axial magnetometermeasurement.

The bottom plot shows the corrected axial magnetic field strength basedon the method flowchart depicted in FIG. 2.1 above. As can be seen fromthe middle plot, the axial magnetometer strength is toolface-dependantand when the tool stops at the minimum or maximum point for staticsurvey, the survey error will be maximized without any dynamiccalibration.

The fitted sine wave can be expressed in SCALE*sin(φ−θ)+OFFSET. Asdescribed below, SCALE, OFFSET and θ (phase shift toolface angle indegrees) of the fitted sine wave may be determined from the axialmagnetometer measurements (i.e., the mean axial magnetic field strengthshown in the middle plot).

By looking at the middle plot of FIG. 3.2, the upper peak value (themaximum) is about −0.6253 Gauss. The lower peak value (the minimum) is−0.627 Gauss. Therefore, the average value over 0-360 deg. is((−0.627)+(−0.6253))/2=−1.2523/2=−0.62615 Gauss. This average valuebecomes the OFFSET of the fitted sine wave.

The peak-to-peak value is (−0.6253−(−0.627))=0.0017 Gauss. The peakvalue is (0.0017/2)=0.00085 Gauss. This peak value is used as the SCALEfactor.

The upper peak value (the maximum=−0.6253) occurs around 53 deg. Thelower peak value (the minimum=−0.627) occurs around 233 deg. The phaseshift angle (in degrees) can be determined by averaging these angles[θ=(53+233)/2=286/2=143 deg.]. The in-phase angle is 143+180=323 deg.And the out-phase angle is 143 deg.

Accordingly, the fitted sine wave is 0.00085*sin(φ−323)−0.62615, whichis an in-phase sine wave (Equation 1).

The correction sine wave is 0.00085*sin(φ−143)−0.62615, which is anout-phase sine wave (Equation 2). This correction can be added to themean axial magnetic field strength (shown in the middle plot) anddivided by 2 to generate the corrected axial magnetic field strengthshown in the bottom plot. This operation (adding the out-phase sinewave) ensures that the sine wave (azimuthal dependency of the axialaccelerometer response) be canceled.

The corrected B_(z) at the toolface φ is expressed as Equation 3 below:B _(z) _(_)corrected(φ)=(B_(z)(φ)+(0.00085*sin(φ−143)−0.62615))/2  Equation 3

The correction sine wave (Equation 2) can be computed while the sensoris rotated. And, the corrected axial sensor value at φ can be determinedby using Equation 3 whether the sensor is rotating or not. Said in otherwords, sensors must be rotated to generate the azimuthal dependencyplot, but once the correction factor is determined, the correction canbe applied anytime while the tool is rotating or not rotating.

In the example described above, a least-computationally-expensivesine-wave fitting algorithm is used. As a result, the fitting is notperfect and some residual errors can be observed in the corrected plot.However, other sine-wave fitting algorithms are available with anexpense of more intensive computation, such as least-square sine wavefitting (three-dimensional search) and/or discreteFourier-transform-based sine-wave fitting. Instead of using sine-wavefitting, a polynomial fitting or triangle-wave fitting (with lessaccuracy/precision) may also be used. These curve fitting methods areknown in the art and this invention is not limited by any particularsine-wave fitting method.

The method described above may be applied to MWD tools, LWD tools, LWDimaging tools, and RSS tools (including strap-down RSS,roll-stabilizer-based RSS, and RSS with non-rotating/slowly-rotatingsections). In addition, the method described above may be used inconjunction with conventional laboratory-performed (shop-performed)static calibration and/or temperature calibrations. Further, fortoolface reference, magnetic toolface, gravity toolface and calibratedgyro toolface may be used. In this document, the word “toolface angle”may be used interchangeably with “roll angle” and/or “azimuthal angle.”

Discrete azimuthal data points may be further filtered along thediscrete toolface axis. As can be seen in FIG. 3.1 on the bottom plot,the data is very noisy and a very noisy sine wave can be recognized. Theidea is to apply a low-pass filter (such as mean filter) in the x-axisdirection on the discredited toolface. After this operation, the bottomplot should look more like a sine wave that can see in the middle plotof FIG. 3.2.

Discrete azimuthal sectors between 4 and 40 (the sector angles between90 degrees and 9 degrees) may be used. For example, 360 discrete valuesof toolface are used in FIGS. 3.1 and 36 discrete values of toolface areused in FIG. 3.2.

Data validity checks may be performed prior to binning and/ordiscretization. For example, if the data point value is very differentfrom the previously computed mean, that data point may be thrown out.

Embodiments of dynamic calibration of axial accelerometers andmagnetometers may be implemented on virtually any type of computerregardless of the platform being used. For instance, as shown in FIG. 4,a computer system (400) includes one or more computer processor(s) (402)such as a central processing unit (CPU) or other hardware processor,associated memory (405) (e.g., random access memory (RAM), cache memory,flash memory, etc.), a storage device (406) (e.g., a hard disk, anoptical drive such as a compact disk drive or digital video disk (DVD)drive, a flash memory stick, etc.), and numerous other elements andfunctionalities typical of today's computers (not shown). The computer(400) may also include input means, such as a keyboard (408), a mouse(410), or a microphone (not shown). Further, the computer (400) mayinclude output means, such as a monitor (412) (e.g., a liquid crystaldisplay LCD, a plasma display, or cathode ray tube (CRT) monitor). Thecomputer system (400) may be connected to a network (415) (e.g., a localarea network (LAN), a wide area network (WAN) such as the Internet, orany other similar type of network) via a network interface connection(not shown). Those skilled in the art will appreciate that manydifferent types of computer systems exist (e.g., workstation, desktopcomputer, a laptop computer, a personal media device, a mobile device,such as a cell phone or personal digital assistant, or any othercomputing system capable of executing computer readable instructions),and the aforementioned input and output means may take other forms, nowknown or later developed. Generally speaking, the computer system (400)includes at least the minimal processing, input, and/or output meansnecessary to practice one or more embodiments.

Further, those skilled in the art will appreciate that one or moreelements of the aforementioned computer system (400) may be located at aremote location and connected to the other elements over a network.Further, one or more embodiments may be implemented on a distributedsystem having a plurality of nodes, where each portion of theimplementation may be located on a different node within the distributedsystem. In one or more embodiments, the node corresponds to a computersystem. Alternatively, the node may correspond to a processor withassociated physical memory. The node may alternatively correspond to aprocessor with shared memory and/or resources. Further, softwareinstructions to perform one or more embodiments may be stored on acomputer readable medium such as a compact disc (CD), a diskette, atape, or any other computer readable storage device.

Dynamic calibration of axial accelerometers and magnetometers has beendescribed with respect to a limited number of embodiments, those skilledin the art, having benefit of this disclosure, will appreciate thatother embodiments may be devised which do not depart from the scope ofdynamic calibration of axial accelerometers and magnetometers asdisclosed herein. For example, curve fitting described throughout thisdisclosure may be solved using software, firmware, FPGA(Field-Programmable Gate Array), hardware (e.g., including analog ordigital circuits), or combinations thereof. Accordingly, the scope ofdynamic calibration of axial accelerometers and magnetometers should belimited only by the attached claims.

What is claimed is:
 1. A method for making survey measurements whileperforming a drilling operation in a subterranean wellbore, the methodcomprising: (a) rotating a drill string in a subterranean wellbore, thedrilling string including an axial survey sensor deployed therein, theaxial survey sensor having an unknown misalignment with a longitudinalaxis of the drill string; (b) calibrating the axial survey sensor by:(i) causing the axial survey sensor to continuously make axial surveymeasurements while rotating in (a) to acquire a data log; (ii) causing adownhole processor to group the axial survey measurements in the datalog into a plurality of discrete azimuthal sectors; (iii) causing thedownhole processor to compute a mean axial survey measurement value foreach of the discrete azimuthal sectors; (iv) causing the downholeprocessor to fit a mathematical function of azimuth to the mean axialsurvey measurement values computed in (iii); (v) causing the downholeprocessor to process the mathematical function to compute a correctionfunction, the correction function being a mathematical function of theazimuth angle; (c) causing the axial survey sensor to acquire an axialsurvey measurement; and (d) causing the downhole processor to combinethe axial survey measurement and the correction function to compute acalibrated axial survey measurement.
 2. The method of claim 1, whereinthe axial survey sensor comprises at least one of an accelerometer and amagnetometer.
 3. The method of claim 1, wherein the axial survey sensoris a part of at least one selected from a group consisting of a MWDtool, a LWD tool, a downhole imaging tool, a downhole motor, and arotary steerable tool.
 4. The method of claim 1, wherein the azimuthangle is based on at least one selected from a group consisting of agravity toolface, a magnetic toolface, and a gyro toolface.
 5. Themethod of claim 1, wherein the axial survey measurement acquired in (c)is a dynamic survey measurement acquired while the drill string isrotating in (a).
 6. The method of claim 1, wherein the axial surveymeasurement acquired in (c) is a static survey measurement acquiredwhile the drill string is not rotating.
 7. The method of claim 1,wherein (b) (iv) comprises causing the downhole processor to fit a sinefunction to the mean axial survey measurement values computed in (iii).8. The method of claim 7, wherein the correction function is a sinefunction.
 9. The method of claim 1, wherein rotating the drill string in(a) drills the subterranean wellbore, the method further comprising: (e)performing the drilling operation in (a) using the calibrated axialsurvey measurement computed in (d).
 10. The method of claim 9, whereinthe calibrated axial survey measurement computed in (d) is used in (e)to make automated steering decisions while drilling in (a).
 11. Themethod of claim 9, further comprising: (f) repeating the calibrating in(b) at a predetermined time interval to compute an updated correctionfunction.
 12. The method of claim 1, wherein the axial surveymeasurements acquired in (c) are automatically calibrated in (d).
 13. Asystem for performing a drilling operation in a subterranean wellbore,the system comprising: a drill string deployed in the subterraneanwellbore, the drill string including a bottom hole assembly (BHA) havingan axial survey sensor configured to acquire axial survey measurementswhile the drill string rotates in the wellbore; a processor deployed inthe BHA, the processor configured to: (i) cause the axial survey sensorto continuously make axial survey measurements while the drill stringrotates to acquire a data log; (ii) group the data points in the datalog into a plurality of discrete azimuthal sectors; (iii) compute a meanaxial survey measurement value for each of the discrete azimuthalsectors; (iv) fit a mathematical function of azimuth to the mean axialsurvey measurement values computed in (iii); (v) process themathematical function to compute a correction function, the correctionfunction being a mathematical function of the azimuth angle; (vi)combine an axial survey measurement and the correction function tocompute a calibrated axial survey measurement; and (vii) use thecalibrated axial survey measurement in a drilling operation.
 14. Thesystem of claim 13, wherein the axial survey sensor comprises at leastone of an accelerometer and a magnetometer.
 15. The system of claim 13,wherein the axial survey sensor is a part of at least one selected froma group consisting of a MWD tool, a LWD tool, a downhole imaging tool, adownhole motor, and a rotary steerable tool.